The earliest cooler may have been a shaded basket. Since then dramatic improvements have been made to the design. These improvements stem from three basic technological advances: Availability of insulating materials, better manufacturing, and the ability to pump heat from liquids or gases which are then used to keep the inside of the container cool.
Insulating materials that are available all reduce heat transfer between their sides, and are built to suit their intended uses. These materials include walls separated by vacuum (to prevent convection) or a solid foam or simply by air. They likewise may include reflective surfaces (to reflect radiated heat).
Construction advances allow the insulating material to fit seamlessly to become a container. Construction also makes coolers practical in a wide variety of uses, despite the relative fragility of insulating materials, because the insulating material is housed in excellent casings.
Cold materials, including most commonly ice, are then put into the cooler to maintain the lower temperature. The cold material will be at a lower temperature than the atmosphere surrounding the cooler. When heat gets through the insulating material into the cooler, raising its temperature, the cold material absorbs that heat. The cooler will therefore reach a stable temperature lying between that of the outside atmosphere and that of the cold material. Where exactly the temperature lies in that range depends on several factors including contents of the cooler, efficacy of the insulating material, and surface areas.
The most effective cold materials are not only cold, but have had the latent heat of a state change extracted from them. For example, to become ice the latent heat of freezing is extracted from water. Thus, the best cold materials change state as they absorb heat Most often this means that they melt, but they may also liquify or even sublimate.
This is only one major problem with ice. It turns back to water, which floods the cooler, and makes it difficult for the remaining ice to keep the cooler cold. The second major problem with ice is that it is not cold enough. That is, because it changes state at 0.degree. Celsius (32.degree. Fahrenheit), this (and only rarely lower) is the temperature that it draws the cooler toward. As explained above, the cooler generally does not reach the temperature of the cold material; and thus, when cooled by ice, the cooler will not keep frozen foods frozen.
The problem that ice turns to water (spillage) has been inadequately solved several ways, including holding the ice within its own container, which reduces its surface area and cooling properties. The relative warmth of ice's freezing point has not been solved.
Colder materials, that is, materials that change state at lower temperatures, obviously will hold the cooler colder. For example, dry ice sublimates at -77.2.degree. Celsius (-107.degree. Fahrenheit). This takes care of any problems with messy melted dry ice spillage but produces CO.sub.2, which is corrosive.
Another problem with dry ice is too much cooling ability. It can drop temperatures below freezing, as follows from the explanation above, which might be colder than desired. A related problem is its uneven cooling; that is, as a block of dry ice, such as that produced by U.S. Pat. No. 5,528,907, which is incorporated by reference for its information on the properties of CO.sub.2, shrinks, its cooling ability drops.
What is needed is a way to control the rate of heat transfer to a block of dry ice, and to use such a block in a cooler in a commercial and practical manner, which allows the cooler to hold a temperature below or above freezing, as desired by its owner. This allows a cooler with separate chambers for frozen and not frozen foods, that is, to perform like a fridge.